The Long Term Evolution-Advanced (LTE-Advanced) next-generation wireless communication system of 3GPP requires the downlink to provide a peak rate of 1 Gps and a peak spectral efficiency of 30 bps/Hz, and this brings about challenges to the physical layer transmission scheme of the system. A multiple input multiple output (MIMO) multi-antenna system supports transmission of parallel data streams, thereby greatly enhancing the system throughput. Under general circumstances, independent forward error-correcting code encoding is firstly performed in parallel data streams transmitted in the multi-antenna system, and the encoded codeword is then mapped to one or more data transmission layers. When the codeword is mapped to plural transmission layers, it suffices to convert the serial data output from the encoder into corresponding plural layers. In one transmission, the number of all layers supported by the system is also referred to as the rank of the transmission. The process of converting the data of each layer into the data of each physical antenna is referred to as the pre-coding process of signals. LTE-Advanced Rel-10 supports the pre-coding technique with the maximum rank of 8.
In order for the receiving terminal to perform MIMO decoding and the associated demodulation, it is necessary for the transmitting side to transmit a pilot sequence, namely a demodulation reference signal (hereinafter referred to as “DMRS”), for estimating channels. Design of DMRSs requires that corresponding DMRSs of data transmission layers be orthogonal to one another, that is, to ensure that equivalent channels to the pre-coded channels of the transmission antennas are free of interference. In the Rel-10 system, corresponding DMRSs of the data transmission layers are differentiated by the frequency division multiplexing (FDM) and/or code division multiplexing (CDM) mode(s). Code division multiplexing is realized by spectrum-spreading sequences with ideal correlation via an orthogonal cover code (hereinafter referred to as “OCC”) sequence. The OCC sequence is usually a Walsh sequence or a discrete Fourier transform (DFT) sequence.
As the inventors found during the process of the present invention, if an OCC sequence is mapped (spectrum-spread) in a time domain, it is usually presumed that channels on the physical resources corresponding to the cover code sequence are identical. Assume that the spread factor of a spectrum-spread sequence is M, it is then considered that channel responses of M number of OFDM symbols are identical. Such assumption is true only in a low-speed motion environment. With the increase in motion speed of a mobile station, the change in channel responses of the M number of OFDM symbols accordingly increases, and orthogonality of the spectrum-spread code is damaged, whereby data transmission layers interfere with one another, and the precision in channel estimation is lowered.
Moreover, in the Rel-10 system, DMRSs are subjected to the same pre-coding treatment as the data, and mapped to transmission antennas. The pre-coding treatment enables the DMRSs corresponding to the code-division multiplexed data transmission layers to be linearly stacked, and when DMRSs corresponding to M number of data transmission layers are stacked in the same direction, a signal with an amplitude of M is obtained; whereas when DMRSs corresponding to M number of data transmission layers are stacked in opposite directions, they counteract one another to obtain a signal with an amplitude of zero. If such power imbalance of each transmission antenna occurs in the entire frequency domain bandwidth, efficiency of transmission power will be markedly lowered.
As should be noted, the above introduction of the background is presented merely to facilitate clear and comprehensive explanation of the technical solutions of the present invention, and to make it easy for persons skilled in the art to comprehend. It should not be considered that these solutions are publicly known to persons skilled in the art only because they have been enunciated in the Background of the Related Art section of the present invention.
Reference documents of the present invention are listed below and herein incorporated by reference, as if they were described in detail in the Description of the present application.
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